Carbonylation of carbonylatable reactants, especially alcohols, in particular methanol, is an industrially important homogeneous catalysed batch reaction. Processes for the manufacture of acetic acid from methanol by carbonylation are operated extensively throughout the world. Acetic acid is used in a variety of applications, among which are in the manufacture of (poly) vinyl acetate, cellulose acetate, acetic anhydride, and acetyl chloride.
The manufacture of acetic acid from methanol and carbon monoxide at high temperature and high pressure was described by BASF as early as 1913. In 1941 BASF developed an improved process using Group VIII metal carbonyls as catalysts for carbonylation reactions. This led to the development of a high-pressure, high-temperature process (70 MPa, 250° C.) with a cobalt iodide catalyst.
Monsanto developed a low-pressure process for the manufacture of acetic acid in the late 1960s with a rhodium iodide promoted catalyst system that demonstrated a pronounced higher selectivity and activity than the cobalt-based process, U.S. Pat. No. 3,769,329. The Monsanto operating conditions in the reactor were milder (3-4 MPa and 180-220° C.) than in the BASF process.
In the early 1980s Celanese Chemical Company developed a low-reaction-water rhodium -catalysed methanol carbonylation process using inorganic iodide salts to improve catalyst stability and activity, U.S. Pat. No. 5,001,259.
BP developed in the early 1990s a process using iridium instead of rhodium in the catalyst system, a process known as the Cativa™ process (Process Engineering, 1996, July, p. 21). Said process is claimed to improve catalyst stability, increase reaction rates, increase yields, and produce less liquid by-products.
Thomas Swan & Co. discloses in WO 01/07388 a continuous process for carrying out carbonylation reactions with carbon monoxide using a heterogeneous catalyst with or without use of a solvent medium, wherein at least one component is under supercritical or near critical conditions.
Ionic liquids have been used as solvents for organic reactions and metal-catalysed reactions including carbonylation reactions.
US 2004/0059153 (Institut Francais du Petrole) discloses a process for liquid phase carbonylation of alcohols by carbon monoxide in the presence of at least one catalyst comprising at least one rhodium and/or iridium complex and a halogenated promoter in at least one non-aqueous ionic liquid. The liquid product of the process must be separated by distillation after decreasing the pressure. Said decrease in pressure may cause more or less pronounced deactivation of the catalyst system used.
U.S. Pat. No. 4,366,259 discloses a high pressure batch process for preparing acetic acid and propionic acid and their esters by contacting a mixture of carbon monoxide and hydrogen gas with a catalyst system comprising a ruthenium-containing compound and a cobalt halide dispersed in a low melting quaternary phosphonium or ammonium base or salt. The liquid product must be separated by distillation.
The liquid phase processes discussed above require separation of the reaction products from the catalysts by distillation from the reactor or by flashing of the reaction solution at reduced pressure. Catalyst decomposition and precipitation may cause problems during the flashing process wherein the liquid products from the reactor are sub-jected to a pressure decrease. Furthermore these separation processes are often cumbersome, require additional reaction steps, and require the use of expensive corrosion-resistant equipment..
US 2003/0212295 (Charles et al.) discloses a continuous process for the preparation of carbonylatable products wherein carbon monoxide, a reactant and a halide in the gas phase are contacted with a non-volatile catalyst solution comprising an ionic liquid and a Group VIII metal to produce a carbonylation product in the gas phase. The process is said to be useful for the continuous preparation of acetic acid by the carbonylation of methanol. The catalytically active component is incorporated in the ionic liquid, which is positioned in the reactor on a metal filter.
A need still exists for an improved process for the carbonylation of carbonylatable reactants, which provides for use of less catalyst materials, presents few requirements for the equipment used and allows a simple process design without the need for recirculation of and pressure change for the catalyst system.